1. Field of the Invention
The invention relates generally to voltage converters and, more specifically, to a high-efficiency, wide input voltage range buck converter.
2. Description of Related Art
Generally, a voltage regulator converts an input voltage to a different output voltage. An example of a typical application is a battery powered electronic device such as a portable computer. In an example such as this, a voltage regulator is required to provide a predetermined and constant output voltage to a load from an often fluctuating input voltage source, the battery.
A buck converter converts the input voltage to a lower output voltage, and a synchronous buck converter includes a pair of switches coupled in series across the input voltage source. One switch, the voltage switch, is coupled to the voltage source and the other switch is connected to ground. An output filter typically including an inductor and a capacitor is connected to a junction formed by the pair of switches for providing the output voltage to a load. A controller drives the switches to connect the output filter to the voltage supply or to ground to maintain the output voltage at a predetermined level.
In many voltage regulator circuits, an N-channel metal oxide semiconductor field effect transistor (MOSFET) is used for the voltage switch to supply current to the output filter and in turn, to the load. An N-channel MOSFET is used rather than a P-Channel MOSFET because the N-channel MOSFET has a lower drain-source resistance when it is an "on" condition (Rds.sub.-- ON). However, it is generally more difficult to drive an N-Channel MOSFET to a low Rds.sub.-- ON condition. To have a low Rds.sub.-- ON condition in an N-Channel MOSFET, its gate must be driven higher than its drain. This requires a separate "step-up" voltage that is about 5 volts higher than the input voltage to ensure that the gate of the N-Channel MOSFET is driven above its drain.
FIG. 1 illustrates a simplified circuit diagram of a prior alt buck converter 10 having an N-channel MOSFET voltage switch 12. A lower switch 14 is coupled in series between the voltage switch 12 and ground. An output filter 16 provides output voltage to a load 18 under the operation of a controller 20. The output filter 16 is connected at a junction between the voltage switch 12 and the lower switch 14. The controller 20 drives the switches 12 and 14 to maintain the desired output voltage level. The input voltage Vin is coupled to a drain 22 of the N-channel MOSFET voltage switch 12. To turn the switch 12 completely on and insure a low Rds.sub.-- ON, an additional element, such as a is charge pump 30 or boost converter, is coupled between the input voltage V.sub.in and the controller 20 to boost the signal provided from the controller to the gate 24 of the voltage switch 12. The charge pump 30 is necessary to drive the gate 24 of the switch 12 higher than the drain 22 sufficiently to fully turn on the switch 12. The charge pump 30 adds cost and complexity to the voltage converter 10, and increases power consumption, which is especially undesirable in a battery powered application.
Alternately, the gate of a P-Channel MOSFET, needs only to be driven at about 4.5 volts lower than its source to ensure a low Rds.sub.-- ON condition. Unfortunately, existing voltage converters utilizing P-channel MOSFET voltage switches have been largely unsatisfactory. These circuits often function only with a limited input voltage range. Further, they tend to be inefficient, which is a fundamental concern with battery powered applications such as mobile computers or portable telephones.
Thus, a need exists for a voltage converter that addresses the shortcomings of the prior art.